This R21 application is focused on the BHD gene ortholog in the fission yeast Schizosaccharomyces pombe (S. pombe). Birt-Hogg-Dube (BHD) syndrome is an autosomal dominant disorder characterized by skin hamartomas, lung cysts, spontaneous pneumothorax, and renal cell carcinoma. A germline mutation in the BHD gene was also recently found in a large kindred with dominantly inherited primary spontaneous pneumothorax (PSP) and bullous lung lesions. The cellular pathway through which inactivation of BHD leads to disease is completely unknown. However, hamartomas also develop in patients with germline mutations in at least four other tumor suppressor genes (PTEN, TSC1, TSC2 and LKB1). Loss of the protein products of these genes results in activation of the kinase mTOR. The S. pombe BHD ortholog encodes a protein with high sequence conservation to human BHD. In published work, we demonstrated that S. pombe with deletion of the tsc1+ or tsc2+ genes have defects in amino acid uptake. In preliminary studies, we generated a BHD mutant in S. pombe, delta-bhd. We found that delta-bhd S. pombe are resistant to a toxic analog of methionine, suggesting decreased methionine uptake. Delta-hbhd S. pombe also have a defect in the leucine uptake, which is similar to delta-tsc1, delta-tsc2 and delta-tor1 mutants. Based on these data, our central hypothesis is that the Bhd protein signals through Tor1 in S. pombe. We propose the following specific aims. Aim 1: We will determine whether Bhd functions in the same pathway as Tsc1, Tsc2, and Tor1 in S. pombe. Aim 2: We will determine the effect of disease causing conserved mutations in the bhd+ gene on leucine and methionine uptake. Aim 3: We will identify proteins in the Bhd pathway in yeast that can rescue the methionine uptake defect using genetic screens. Our long- term goal is to use this S. pombe model to elucidate the functions of the human BHD protein. This may provide novel insights into cellular pathways involved in the pathogenesis of cystic lung diseases. Lay summary. Yeast models can provide key information about the most important cellular functions of human disease-associated genes. Birt-Hogg-Dube (BHD) syndrome is a rare disease associated with lung cysts, lung collapse, and kidney cancer. Nothing is currently known about how BHD gene mutations lead to human disease. We will use a novel yeast model of BHD to elucidate the function of the BHD protein.